A recent significant advancement in the field of gas lancing is the development of the coherent jet technology disclosed, for example, in U.S. Pat. No. 5,814,125—Anderson et al. and in U.S. Pat. No. 6,171,544—Anderson et al. In the practice of this technology one or more high velocity gas jets ejected from one or more nozzles on a lance are maintained coherent over a relatively long distance by the use of a flame envelope around and along the high velocity gas jet(s). The flame envelope is formed by combusting fuel and oxidant ejected from the lance from one or more rings of ports around the nozzle(s). The fuel and oxidant combust under atmospheric pressure conditions to form the flame envelope. Operation of this technology under subatmospheric pressure conditions would be desirable. However, it is problematic because of the negative effects of flame lift-off and blowout experienced under such conditions.